1. Field of the Invention
The invention relates to a method of and a circuit arrangement for producing a gamma corrected video signal.
2. Description of the Prior Art
Display tubes for television introduce a distortion of the picture information due to a non-linear relationship between the applied signal voltage and the resulting screen brightness. This relationship may be approximated by the expression EQU Lo-K V.sub.s .delta.
where
Lo=Light output PA1 K=constant PA1 V.sub.s =applied signal voltage PA1 .delta.=constant, known as gamma. PA1 V.sub.s =output signal for transmission PA1 V.sub.i =input signal requiring modification PA1 .delta.=constant, as before.
Typically .delta. has a value of 2.5 whereas ideally it would be 1.0.
To prevent this distortion becoming apparent to the viewer the video signal is transmitted in a modified form by passing the signal through a gamma correction circuit which introduces a complementary distortion.
The modification carried out by a gamma corrector may be described by the expression EQU V.sub.s =V.sub.i.sup.1 /.delta.=V.sub.i.sup.G
where
In practice the value of .delta. employed in the gamma correction circuit may not be exactly 2.5 but chosen to give the best subjectively pleasing result.
It is sometimes necessary, for example in television cameras, to perform the inverse operation to derive an uncorrected signal from a previously gamma corrected signal. In this specification the term gamma correction is to be understood to include both the correction of an uncorrected signal and the derivation of an uncorrected signal from a corrected signal.
Gamma correction is normally achieved by first converting the input signal into its logarithm, then multiplying this signal by the desired correction factor G, and finally applying the resultant signal to an exponential or anti-logarithm converter. Such an arrangement is disclosed in an article entitled "Transistorised Non-Linear Function Generation" by P. Kundu and S. Banerji which was published in Industrial Electronics, January 1964 at pages 35 to 41.
If the signals are expressed in digital form, as is increasingly common, the same approach can be followed except that logarithmic and exponential conversion may then be achieved by means of `look-up` tables stored in programmable read only memories (PROMs). Some difficulty is encountered, however, with the multiplication process which must be performed on each digital sample within the sample period, typically 75 ns. The digital signal, after conversion into its logarithm, may be 12 bits wide and the correction coefficient, G, 6 or more bits wide. The multiplication of a 12 bit number by a 6 bit number within 75 ns entails either complex circuitry to form and add partial products, or the use of integrated circuit multipliers which consume considerable power and are relatively expensive.